The present invention relates to optical devices enabling the recording of data along a track prerecorded on the surface of a disc-shaped carrier. The date writing operation is performed by means of an optically modulated beam of radiation which projects a spot of small size on the disc surface. The prerecorded track is formed by concentric turns of which the spacing is such that the spot may illuminate only a single turn. A spot having dimensions of the order of one micron is obtained by focussing a laser beam with a lens, which renders it possible to secure a high power density for data writing and, thanks to the closeness of the turns, a high-density storage facility. In view of the small spot size, the optical properties of the prerecorded surface largely depend on diffraction phenomena. Thus, when a smooth surface of the disc is illuminated, the reflection of light or the transmission of light occur with a solid angle which corresponds to the angular aperture of the lighting beam. By intercepting the light emerging from a smooth illuminated surface, a maximum of illumination may be observed in a detection plane. On the contrary, if the spot illuminates a disc area comprising a surface irregularity, the solid angle with which the reflected or transmitted radiation emerges may be wider than the angular aperture of the lighting beam. The detection of a surface irregularity may be made apparent by a darkening effect, since a proportion of the emergent radiation is not intercepted by the detector system.
In the case of optical write-read devices utilising a prerecorded data carrier, a surface relief comprising a low level and a high level is adopted for the prerecorded track. A central cross-section of the prerecorded disc displays an alternation of depressions and prominences defining two imbricated grids, the one comprising the track elements and the other the track interstices. The track interstices bordering a track element form a structure which must be followed by the spot whilst the latter remains as centred on the track as possible. To this end, a radial control is provided in order to tend to cancel out any deviation from the track. The detection system should be able to establish a clear distinction between the track and the two track interstices bordering the same. This distinction is based on a contrast of intensity which does not depend on the relief direction but on the radial dimensions given to the surface irregularities as compared to the lighting spot. The data recording action commonly has the result of altering the optical characteristics of the track, but this alteration may cause defective operation of the system for the detection of deviations in following the track.
With a view to overcoming this disadvantage, the invention provides a particular dimensioning of the prerecorded structure which in predetermined lighting conditions provides an accentuation of the contrast between the track and the "pretracks" by virtue of recording the data.